Recently, various research groups have been studying techniques for evaluating the health conditions of people based on their vital signs. As the results, some researchers found the correlation between the health conditions of the people and their vital signs, and proposed methods for evaluating the health conditions of people based on their vital signs at academic meetings. However, such correlation analysis was performed based on changes in vital signs observed in segments where the health conditions surely changed, and thus these results are mere results obtained at laboratories far different from real environments. At present, there is no established technique for performing back calculation to evaluate the health condition of a person in the case where a change in a vital sign is observed. One technique proposed in view of these problems as a method for discriminating a specific health condition of a person in the case where the person has a stress from a specific health condition of a person in the case where the person is doing sports is a technique for measuring a pulse wave, calculating pulse wave values, the upper peak values of the pulse wave, and the pulse pressure values each of which is the difference between an upper peak value and a corresponding lower peak value of the pulse wave, and determining a current status from among stress, sleepiness, fatigue (muscle fatigue) (See Patent Reference 1, for example).
FIG. 1 is a block diagram showing the structure of a health condition discrimination device disclosed in Patent Reference 1. The health condition discrimination device includes a pulse wave measurement unit 1301, a calculation unit 1302, a discrimination unit 1303, and an alert output unit 1304.
The pulse wave measurement unit 1301 measures the flow amount of haemoglobin carried by oxygen and the flow amount of haemoglobin not carried by oxygen which are generated through blood circulation. The calculation unit 1302 calculates data of pulse values, blood pressure values, and pulse pressure values each of which is the difference between the upper and lower peaks of a corresponding one of the pulse waves, based on the pulse wave data measured by the pulse wave measurement unit 1301. The discrimination unit 1303 detects changes in the data calculated by the calculation unit 1302, and discriminates the health conditions into plural levels. The alert output unit 1304 outputs alerts according to the levels discriminated by the discrimination unit 1303.
As mentioned above, there are attempts to detect pulse waves from human bodies using optical or piezoelectric pressure sensors, and determine human health conditions. However, pulse waves are susceptible to body movements or the like, and thus there is a need to discriminate between pulse wave signs and noise. Exemplary conventional methods for detecting noise included in pulse waves include a method for discriminating noise based on the variance in the peak values of velocity plethysmogram (abbreviated as VPG) obtained by differentiating the detected pulse waves (See Patent Reference 2, for example).    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2003-61921 (page 8, FIG. 2)    Patent Reference 2: Japanese Patent Publication No. 3835073